1. Field of the Invention
This invention relates generally to integrated circuit fabrication and, more particularly, to masking techniques.
2. Description of the Related Art
As a consequence of many factors, including demand for increased portability, computing power, memory capacity and energy efficiency in modern electronics, integrated circuits are continuously being reduced in size. To facilitate this size reduction, the sizes of the constituent features, such as electrical devices and interconnect line widths, that form the integrated circuits are also constantly being decreased.
The trend of decreasing feature size is evident, for example, in memory circuits or devices such as dynamic random access memories (DRAMs), static random access memories (SRAMs), ferroelectric (FE) memories, etc. To take one example, DRAM typically comprises millions of identical circuit elements, known as memory cells. In its most general form, a memory cell typically consists of two electrical devices: a storage capacitor and an access field effect transistor. Each memory cell is an addressable location that can store one bit (binary digit) of data. A bit can be written to a cell through the transistor and read by sensing charge on the storage electrode from the reference electrode side. By decreasing the sizes of constituent electrical devices and the conducting lines that access them, the sizes of the memory devices incorporating these features can be decreased. Additionally, storage capacities can be increased by fitting more memory cells into the memory devices.
The continual reduction in feature sizes places ever greater demands on techniques used to form the features. For example, photolithography is commonly used to pattern features, such as conductive lines, on a substrate. The concept of pitch can be used to describe the size of these features. Pitch is defined as the distance between an identical point in two neighboring features. These features are typically defined by spaces between adjacent features, which are typically filled by a material, such as an insulator or conductor. As a result, pitch can be viewed as the sum of the width of a feature and of the width of the space separating that feature from a neighboring feature. Due to factors such as optics and light or radiation wavelength, however, photolithography techniques each have a minimum pitch below which a particular photolithographic technique cannot reliably form features. Thus, the minimum pitch of a photolithographic technique can limit feature size reduction.
“Pitch doubling” is one method proposed for extending the capabilities of photolithographic techniques beyond their minimum pitch. Such a method is illustrated in FIGS. 1A-1F and described in U.S. Pat. No. 5,328,810, issued to Lowrey et al., the entire disclosure of which is incorporated herein by reference. With reference to FIG. 1A, photolithography is first used to form a pattern of lines 10 in a photoresist layer overlying a layer 20 of an expendable material and a substrate 30. As shown in FIG. 1B, the pattern is then transferred by an etch step (preferably anisotropic) to the layer 20, forming placeholders, or mandrels, 40. The photoresist lines 10 can be stripped and the mandrels 40 can be isotropically etched to increase the distance between neighboring mandrels 40, as shown in FIG. 1C. A layer 50 of material is subsequently deposited over the mandrels 40, as shown in FIG. 1D. Spacers 60, i.e., material extending or originally formed extending from sidewalls of another material, are then formed on the sides of the mandrels 40 by preferentially etching the spacer material from the horizontal surfaces 70 and 80 in a directional spacer etch, as shown in FIG. 1E. The remaining mandrels 40 are then removed, leaving behind only the spacers 60, which together act as an etch mask for patterning underlying layers, as shown in FIG. 1F. Thus, where a given pitch formerly included a pattern defining one feature and one space, the same width now includes two features and two spaces defined by the spacers 60. As a result, the smallest feature size possible with a photolithographic technique is effectively decreased.
It will be appreciated that while the pitch is actually halved in the example above, this reduction in pitch is conventionally referred to as pitch “doubling,” or, more generally, pitch “multiplication.” That is, conventionally “multiplication” of pitch by a certain factor actually involves reducing the pitch by that factor. The conventional terminology is retained herein.
The critical dimension of a feature is the feature's minimum dimension. For features formed using the spacers 60, the critical dimension typically corresponds to the width of the spacers. The width of the spacers, in turn, is typically dependent upon a thickness 90 (see FIGS. 1D and 1E) of the layer 50. Thus, the layer 50 is typically formed to a thickness 90 corresponding to the desired critical dimension.
The quality and uniformity of the spacers 60 directly affect the quality of the integrated circuits partially defined in the substrate 30 using the spacers as a mask. Where the desired spacers 60 are relatively wide compared to the mandrels 40 and/or the space separating the spacers 60, however, it has been observed that the resulting spacers 60 and the etch mask resulting from the spacers 60 can have poor uniformity. This poor uniformity, in turn, can cause poorly defined and non-uniform features to be formed in the substrate. As a result, the electrical performance of integrated circuits formed in the substrate may be degraded or the integrated circuits may be unusable.
Accordingly, there is a need for methods of forming etch masks having highly uniform and well-defined patterns, especially in conjunction with spacers formed in pitch multiplication.